1. Field of the Invention
The present invention relates to a multi-wheel-driving vehicle provided with not less than six drive wheels. More particularly, the present invention relates to a technique for improving the braking capacity of the multi-wheel-driving vehicle, as well as compactness and cost savings in its construction.
2. Background Art
Conventionally, there is a multi-wheel-driving vehicle having three or more axles disposed in parallel, each of which is provided at its left and right ends with respective drive wheels. Generally, each axle is divided into left and right halves which are differentially connected with each other through a differential.
U.S. Pat. No. 4,050,534 discloses a power transmission system for such a multi-wheel-driving vehicle wherein engine power is distributed among three axles as follows. The torque output from a transmission is firstly transmitted to a tandem axle mechanism, i.e., a center differential which differentially connects a pair of coaxial first and second transmission shafts. The tandem axle mechanism distributes a part of the torque to the frontmost (steerable) axle and the rearmost axle through the first transmission shaft and the remainder of the torque to the middle axle (second rear axle) through the second transmission shaft, thereby nicely balancing the torque among the three axles. However, in such a cited conventional power distribution structure, the frontmost axle for steerable front drive wheels is drivingly synchronized with the rearmost axle for unsteerable rear drive wheels, so that the driving of the front wheels is restricted while turning by the driving of the rear wheels, thereby hindering a smooth steering of the vehicle. For avoiding such a problem, it is effective to have the axle of the steerable wheels drivingly differentially connected with the other axles.
Furthermore, this cited document does not disclose an arrangement of brakes. In the cited art, if only the middle axle of the three is provided thereon with a brake, the tandem axle mechanism prevents the braking force applied thereon from being effectively transmitted to the frontmost and rearmost axles. On the other hand, if a brake is just provided on either the front axle or the rearmost axle, the braking force applied on the axle is transmitted to the other of the front or rearmost axle because the two axles interlock with each other through the first transmission shaft. However, the braking force is not effectively transmitted to the middle axle interlocking with the second transmission shaft. Thus, to effectively stop the vehicle, the middle axle and at least one of the frontmost and rearmost axles need respective brakes, whereby at least two brakes are necessary.
Furthermore, as mentioned above, each axle is generally divided into two halves differentially connected with each other. If only one of the halves is provided thereon with a brake, the braking force cannot be effectively transmitted to the other half. If the first and second transmission shafts of the tandem axle mechanism are locked together and if the halves of each axle are locked together, a braking force generated by fewer brakes can be effectively transmitted to all axles, thereby improving compactness of the vehicle and increasing cost-savings. It is preferable that such differential-locking operations are automatically performed due to the driver's braking operation so as to facilitate the driver's work.